# How can two solutions have different ORP values and yet the same pH value?i.e. isn't pH the measure of the OH-'s, i.e. hydroxyls, in a solution, and isn't this the measure of the extra/free...

How can two solutions have different ORP values and yet the same pH value?

i.e. isn't pH the measure of the OH-'s, i.e. hydroxyls, in a solution, and isn't this the measure of the extra/free electrons available [for redox], i.e. the ORP?

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### 1 Answer

Redox potential value depends upon the willingness or ease with which some species accepts (or releases) electrons, while pH is negative logarithm of H+ (or OH-) ion concentration in solution. For systems that does not involve H+ (or OH-) ions, the redox potential of it does not depend upon pH. e.g. Fe3+/Fe2+ has a potential of 0.77V, independent of pH. But for redox systms that involve transfer or dischrge of H+ or OH- ions, redox potential is pH dependent. O2 + 2H2O + 4e- gives you 4OH- has a redox potential of 0.40V at 1M conc, at other concentrations, the potential value changes according to the Nernst equation: E = E0 - .059/n log [red]/[ox]. So for systems where OH- is the only species undergoing redox change, your argument stands good, and definitely concentration of OH- governs pH and also governs the ORP value, albeit through a different set of mathematical operator. For other systems exact relationship between H+ (or OH-) ions and the actual redox process has to be settled first and then an equation interrelating pH and ORP can be arrived at.